Method of making an interposer sub-assembly in a printed wiring board

ABSTRACT

The details of a printed wiring board (PWB) sub-assembly and the method of producing the same are described. The sub-assembly comprises a printed circuit board electrically joined through a plurality of connections to one or more area array devices, such as modules or printed wiring boards. The sub-assembly can serve as a part of an original assembly. The sub-assembly can function as an after market item that can be readily substituted as a replacement for a failed component wherein the dimensional space between the printed circuit board and one or both of the area array devices must provide sufficient clearance for surface mounted devices.

FIELD OF THE INVENTION

[0001] This invention relates to electronic sub-assemblies, andparticularly to structures comprising a printed wiring board laminatethat is electrically and mechanically connected through an interposerconnector to an area array device, such as another board, a module orother active or passive device.

BACKGROUND OF THE INVENTION

[0002] The mechanical and electrical core of a computer system comprisesa plurality of printed wiring boards interconnected with other boards ormodules and with other active or passive devices, such as diodes,semiconductors, capacitors and resistors. The success or failure of suchcomputer systems is dependent upon, among other factors, their abilityto operate without mechanical breakdowns and electrical failures.

[0003] One type of computer system is a microprocessor called a networkserver. A principal function of a network server is to achieve organizedchannels of communication between a plurality of personal computers. Italso serves to house many programs that it shares with the personalcomputers. Thus, the reliable operation of the server is critical to theoperation of an entire network of such personal computers.

[0004] The interconnection between the mating surfaces of a printedwiring board and a module or other active or passive device may beprovided through an interposer. An interposer is a structure thatprovides electrical contact between two such devices (hereinafterreferred to as area array devices). These area array devices typicallyare approximately parallel. The electrical connection between an areaarray device and a printed wiring board is achieved by pressing thedevice and the wiring board together with the thin conductive interposerbetween the two. The interposer can include compressible conductiveelastomers, coil springs or leaf springs to establish the electricalconnection between the wiring board and the other device.

[0005] The need for area array connectors is growing due to the increasein inputs and outputs on area array modules. An area array connector isa type of high-density, low inductance socket available from a number ofsuppliers. These connectors may be any one of a number of differenttypes. One such socket uses compressible ‘fuzz buttons’. These arecompressible wadded wires described, for example, in the followingpatents: U.S. Pat. No. 5,552,752; U.S. Pat. No. 5,146,453 and U.S. Pat.No. 5,631,446. These are small, irregularly wound and inter-twined padsor balls that are made of gold plated beryllium copper wool or goldplated molybdenum wire. These wadded wire balls are compressed in holesin the interposer, which is a thin sheet of insulative material thatseparates the printed wiring board from the area array device. The holesare arranged in a pattern that matches a pattern of conductive pads onthe surfaces of the printed wiring board and the area array device incontact with the interposer. In the assembly process, the ‘fuzz buttons’are compressed between the conductive pads, thereby providing electricalconnections between the pairs of pads on the two surfaces that areseparated by the interposer. Among the other types of connectors aremetal filled elastomers, such as those sold by Tyco Inc. (formerlyThomas & Betts) as Metal Particle Interconnect Elastomers. Metal springsare also used. These metal springs generally are leaf springs having anumber of geometries, such as C-shaped or V-shaped. Soldered connectionscomplicate the disassembly or separation of the printed wiring boardfrom the area array modules if repair or replacement is required.

[0006] A limitation of this type of interconnect technology is theinherent failure rate that can be obtained while placing electricalcontacts in series. The intrinsic failure rate of electrical contacts inseries can be approximated by:

[0007] Failure rate=n·IFR, wherein

[0008] Failure Rate equals the contacts in series

[0009] n=number of contacts in series

[0010] IFR=intrinsic failure of each contact interface.

[0011] Therefore, addition of more electrical contacts in seriestypically increases the failure rate of an electrical circuit.

[0012] Land grid area array interposers have been described in which aninterposer is designed for electrically connecting an electrical deviceto a printed circuit board wherein the interposer has been fabricated toprovide adequate stand-off between the electrical device and the printedcircuit board to provide space for active and/or passive devices.

[0013] To provide a connection between a printed wiring board andanother area array device (which may be a second printed wiring board)using a land grid array (LGA) connector, a space of about 3 mm is neededto allow clearance for any active or passive devices, such as diodes,capacitors and capacitors that are mounted on the surface of the board.An option is to request the vendors of the connector to devise a customconnector that provides the needed height. As is well recognized, thecustomization of any piece of hardware or software can add appreciablyto the overall product development schedule, as well as the cost ofassembly and/or use.

BRIEF DESCRIPTION OF THE INVENTION

[0014] The present invention relates to an electronic sub-assembly andits method of manufacture. The sub-assembly comprises a printed wiringboard (PWB) having a first surface including a plurality of viasterminating at electrical contacts at said surface. These contactsgenerally are in the form of contact pads. A first area array devicehaving a surface, including a plurality of electrical contacts, isspaced from the PWB. A plurality of connectors electrically couple theelectrical contacts on the surface of the PWB to the electrical contactson the surface of the first area array device. The printed wiring boardtypically has a second surface that includes a plurality of electricalcontacts that are coupled to the second surface of the PWB by aplurality of connectors. The vias extend through the PWB to formelectrically conductive plated through holes.

[0015] In one embodiment, an interposer separates the first surface ofthe printed wiring board from the surface of the first area arraydevice. A second interposer may also separate the second surface of thePWB from the second area array device. Each of the interposers includesthe connectors that couple said printed circuit board to the respectivearea array device. The area array devices may be spaced a fixed minimumdistance from the printed wiring board, preferably a distance of about 3mm. The sub-assembly preferably uses connectors that are selected fromthe group consisting of compressible wadded wire contacts, metalsprings, filled conductive elastomers, solder balls and hard solderballs.

[0016] For ease of repair and replacement, the connectors arecompressible wadded wire having a first end soldered to said contactpads and a second end compressed against the electrical contacts on thesurface of the area array device. This allows the area array device tobe easily detached from the sub-assembly.

[0017] Means are also provided for maintaining a fixed minimum spacingbetween the PWB and each area array device

[0018] The invention also relates to a printed wiring boardsub-assembly. The printed wiring board has a first surface containingone or more vias extending from the first surface into the board, and aconductive contact pad on the surface of the printed wiring boardelectrically joined to each of said vias. An interposer structure has afirst surface facing the first surface of the printed wiring board, anda second surface. The structure includes a pattern of holes extendingtherethrough that correspond to the holes in the printed wiring board.Wadded wire electrical contacts are pressed into each hole in thepattern. Each of the electrical contacts has a first end that issoldered or is pasted to a contact pad on the first surface of theprinted wiring board. The second end of the wadded wire contact iselectrically coupled by compressive contact to a conductive lead on asurface of a module or another printed wiring board.

[0019] At least some of the vias typically extend through the board toform plated through holes that terminate at a second board surface. Asecond interposer structure can be joined to contact pads on the secondsurface in a manner similar to that of the first interposer to providean interconnect to a second area array device.

[0020] In yet another embodiment, the invention relates to an electronicsub-assembly that comprises a PWB spaced a fixed minimum distance froman area array device, and the method of making the same. The printedwiring board has a first surface including a plurality of vias extendingfrom the surface into the board. A plurality of electrical contact padsare connected to each of the vias, at least some of the contact padsbeing offset from the corresponding vias. The area array device has asurface including a plurality of electrical contacts. The surface of thePWB and the surface of the area array device are spaced a fixed minimumdistance, preferably about 3 mm, from one another. A plurality ofconnectors electrically couple the contact pads on the surface of theprinted circuit board to the electrical contacts on the surface of thearea array device. The connectors are selected from the group consistingof compressible wadded wire contacts, metal springs, filled conductiveelastomers, solder balls and hard solder balls. The hard solder ballstypically are composed of copper balls coated with a layer containingtin. If normal solder balls are used, they may be embedded in anunderfill material filling the space between the printed wiring boardand the area array device. The wadded wire connectors, filled elastomersand metal springs typically are used by placing them in holes extendingthrough the interposer between the surface of the PWB and thecorresponding surface of the area array device. The interposer may alsocomprise a pair of plastic caps, each cap including a lip extending overan edge of the PWB.

[0021] In yet another embodiment, an electronic sub-assembly comprises aprinted wiring board having first and second generally planar surfacesand a plurality of vias terminating in electrical contact pads on thesurfaces. A first area array device has a generally planar surfacefacing the first generally planar surface of the printed wiring board.The surface of the array device has a plurality of electrical contactsthereon. A plurality of connectors electrically couple the electricalcontacts on the first surface of the printed circuit board to theelectrical contacts on the surface of the first area array device. Theseconnectors can be wadded wire contacts, metal springs, filled conductiveelastomers, solder balls or hard balls or combinations thereof. A firstinterposer separates the first surface of the printed wiring board fromthe surface of the first area array device a fixed minimum distance ofabout 3 mm. The interposer contains a pattern of holes corresponding inlocation to the contacts on the first surface of the printed wiringboard and the connectors are positioned in these holes. The sub-assemblyfurther includes a second area array device having a surface including aplurality of electrical contacts for electrically coupling the secondsurface of the printed circuit board to the second area array device. Asecond interposer separates the second surface of the printed wiringboard from the surface of the second area array device. As with thefirst interposer, the second interposer contains a pattern of holes.These holes correspond in location to the contacts on the second surfaceof the printed wiring board. A plurality of connectors are positioned inthe pattern of holes in the second interposer for electrically couplingthe electrical contacts on the second surface of the printed circuitboard to the electrical contacts on the surface of the second area arraydevice. At least some of the electrical contacts on one or both of thegenerally planar surfaces of the printed wiring board comprise contactpads that are offset from the corresponding vias in the printed wiringboard to which the contact pads are electrically connected. At leastsome of the connectors in the holes of said interposers have a first endsoldered or pasted to contact pads on a surface of the printed wiringboard, while the second end is pressed against the electrical contactson the generally planar surface of the corresponding area array devicewithout soldering or pasting. The invention also contemplates that thesecond interposer may separate the second surface of the printed wiringboard a fixed minimum distance of about 3 mm from the surface of thesecond area array device.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a circuit board assembly using an interposer with waddedwire electrical contacts compressed between two circuitized substratesaccording to the teachings of the present invention;

[0023]FIG. 2 shows a similar assembly with two interposers with waddedwire connections between three circuitized substrates;

[0024]FIG. 3 shows a circuit board assembly using wadded wire contacts,and an underfill material;

[0025]FIG. 4 shows another circuit board assembly using a stand-offinterposer;

[0026]FIG. 5 is a planar view of the interposer and printed circuitboard of FIG. 4;

[0027]FIG. 6 shows another circuit board assembly with C-springconnectors and a ball grid array using hard electrically conductiveballs; and

[0028]FIG. 7 shows another circuit board assembly using conductiverubber contact pads.

DETAILED DESCRIPTION OF THE INVENTION

[0029] Land grid area array interposers have been described in which aninterposer is designed and fabricated for electrically connecting anelectrical device to a printed circuit board wherein the interposer hasbeen specifically designed to provide adequate stand-off between anelectrical device and a printed wiring board to provide space for activeand/or passive devices mounted thereto.

[0030] Turning to the drawings for greater detail, FIG. 1 shows acircuit board assembly comprising a printed wiring board 110electrically connected to an area array device 112, such as a printedcircuit board or module. The printed wiring board 110 includes aplurality of vias 120 extending from the surface 124 into the board.Conductive pads 122 are soldered, pasted or plated to the openings ofeach via 120 on the top surface 116 of the PWB 110, and pads 126 aresoldered, pasted or plated to the bottom surface 124 of the PWB 110.

[0031] An interposer sheet 130 is provided with holes 132 positionallycorresponding to the vias 120 on the bottom surface 124 of the PWB 110.Each hole 132 is filled with wadded wire 134. One end of the waddedwires in the interposer sheet 130 is placed in contact with the contactpads 126 on the bottom surface 124 of the PWB and is soldered thereto.

[0032] The area array device 112 contains electrical contacts orinterfaces 148 on the board surface facing the PWB 110. The contacts arepositioned to abut the other end of the wadded wire contacts 134. Thewadded wire contact is compressed to the plated through hole 120 to forman electrical contact from the pads 148 on the module 112 to the platedthrough hole 120 using a conventional clamping device of the typecommonly employed in the art. For purposes of illustration, the clampingpressure in FIG. 1 is shown by opposing arrows 100 and is capable ofevenly distributing clamping pressure to the mating surfaces of the PWB110, the area array device 112, and the two surfaces of the interposersheet 130. The clamping means 100 can, for example, comprise severalC-clamps placed around the edges of the mating surfaces sandwichedbetween backing plates (not shown). Instead, a plurality of bolts canextend through holes in the various layers of the sandwich and throughbacking plates, and can be secured by nuts to provide the clampingpressure.

[0033] In accordance with one embodiment of the invention, the solder isthen reflowed to form the electrical connection between the contact pad126 and the wadded wire 134 in the interposer 112. This then comprisesthe interposer sub-assembly 144. However, the interface between thewadded wire and the contact pad 148 is not soldered and, thus, remainsseparable, relying only on the clamping pressure to maintain electricalcontact. This mechanical connection then facilitates the disassembly ofthe wiring board 110 and the area array device 112.

[0034] Referring now to FIG. 2, an interposer sub-assembly 244 comprisesan inner PWB 210 containing a plurality of vias 220 extending betweenthe surfaces of the PWB to form plated through holes (PTH)s) 218. ThesePTHs join conductive pads 222 on the upper surface 216 of the PWB 210 tocorresponding conductive pads 226 on the bottom surface 224 of the PWB210. The upper pads 222 contact fuzz buttons 234 pressed into holes 242in interposer 240. The fuzz buttons, in turn, make electrical contactwith contact pads 246 on the upper area array device 214. The pads 226on the lower surface 224 of the PWB 210 are electrically andmechanically coupled to a lower area array device 212 through fuzzbuttons of wadded wire 234 compressed into holes 232 in the interposersheet 230. The electrical connections from wadded wire 234 to theconductive pad 226 and the wadded wire 234 to conductive pad 222 isaugmented with solder, conductive paste, or other permanent connectionto improve control reliability or improve handling by reducing thenumber of loose pieces. On the other hand, mechanically coupling thesub-assembly 244 to the area array devices without solder or pastefacilitates disassembly for repair and replacement. Another advantage ofthis system is increased design flexibility with minimum inventory.Modifications can be made in the thickness of the printed circuit board210 to allow the same interposers 230, 240 to accommodate multiplestandoff heights and/or component heights on devices 212, 214.

[0035] It should be understood that the invention can also be practicedby replacing the fuzz buttons 234 with springs, such as C-springs madeby Intercon Corporation, or by the use of electrically conductivepolymers or solder balls or other means for effecting an electricalconnection between the PWB and the devices that are spaced from the PWB.

[0036] The present invention also relates to the use of a standardheight connector to get a fixed minimum space between the printed wiringboard and the area array device. A 3 mm gap is adequate for manystandard devices used in the industry today. The PWB is a laminatestructure that can be soldered to a daughter card using establishedprocesses. The contact pads have a dog-bone structure on both the topand the bottom surfaces of the board, and these pads are electricallyjoined to the plated through holes. The plated through holes can be assmall as 0.010″ for an interposer thickness up to 0.110″. For greaterthickness, larger PTHs are required.

[0037] Turning now to FIG. 3, a PWB 310 is shown between a lower areaarray device 312 and an upper area array device 314. The PWB 310 isshown as a laminated structure comprising conductive metal layers 328embedded in a prepeg. The PWB contains a plurality of vias extendingthrough the PWB to form plated through holes 318 joining the uppersurface 316 of the PWB with its lower surface 324. The PTHs 318 arejoined to conductive pads 322 on the upper surface 316 and tocorresponding conductive pads 326 on the bottom surface 324. The pads322 are offset from the vias 318 in a so-called dog bone configuration.This permits the use of enlarged pads to establish electrical contactbetween the two surfaces of the PWB and the corresponding surfaces ofthe modules. The upper pads 322 contact solder balls 360 which, in turn,make electrical contact with the upper area array device 314. The solderballs are supported by use of an underfill material 370 in accordancewith well established practices. The underfill serves to reduce thecreep of the solder balls under compression load and improves thefatigue life of the solder balls due to thermal strains. The pads 326 onthe bottom surface 324 of the PCB are electrically connected to thelower area array device 312 through fuzz buttons 334 compressed intoholes 332 in the interposer 330 in the manner previously described. Ascan be seen from the drawing, with the use of the dog bone configurationfor the pads, it is not necessary to have an exact alignment between thesolder balls, the plated through holes and the fuzz buttons. It shouldbe understood that solder balls, metal springs or other connectors canbe substituted for the fuzz buttons in the interposer.

[0038] To avoid the need to use an underfill material to controlmovement of the solder balls, a plastic comb structure as shown in FIGS.4 and 5 may be used. This structure provides a physical limit between aPWB 410 such as a mother board and a mezzanine device 414. This plasticcomb structure comprises a pair of plastic caps 480 to prevent thecollapse of the solder balls 460 between the device 414 and the printedwiring board 410 when the components are clamped together. Each capincludes a lip 482 extending downward over the edge of the PWB. The lipsserve to accurately position the interposer with respect to the PWB andto restrict relative movement between these two components. The capsinclude a plurality of semi-circular cutouts 484 corresponding to theouter row of solder balls 460. The thickness of the caps is at least asgreat as the diameter of the solder balls. Thus, when the assembly isclamped together, the balls are not flattened or damaged by thecompressive forces. Furthermore, the caps 480 allow the area arraydevice 414 to be removed for purposes of rework or replacement.

[0039] The PWB 410 contains several conductive layers 428 laminatedtherein, and a plurality of plated through holes 418 extendingtherethrough. These holes 418 connect with conductive pads 426 on thebottom surface 424 of the PWB 410. An interposer sheet 430 contacts thebottom surface 424 and contains a plurality of holes 432 into whichwadded wire fuzz buttons 434 are pressed. The dog-bone shape of the pads422, 426 permits the solder balls 460 and wadded wire 434 to makeelectrical contact through the conductive PTHs, even though the solderballs and the wadded wire are not located directly above or below theplated through holes. The caps 480 typically are added after the PWB 410and the top module 414 are soldered together and cleaned. Thus, even ifthe two members creep toward one another, there is a set limit, asdetermined by the thickness of the cap, that will keep them apart andwill avoid an electrical short between the solder balls.

[0040]FIG. 6 shows an arrangement wherein the deformable solder ballsare replaced by hard electrically conductive balls composed of, forexample, copper cores plated with a surface layer of tin or a tin alloy.The sub-assembly 644 comprises a printed wiring board 610, an array ofhard electrically conductive solder balls 662, and an upper device 614as another printed wiring board or a module. The PWB 610 contains aplurality of plated through holes 618 connected to a plurality ofconductive pads 622 arranged in a dog-bone pattern on the upper surface616 and to another set of offset pads 626 on its bottom surface 624. Aspreviously noted, this dog-bone arrangement of the pads eliminates thenecessity of placing the hard solder balls directly over the PTHs andthe C-springs directly beneath the PTHs. The hard balls 662 complete theelectrical contacts between the pads 622 and the module 614. The hardballs 662 resist collapse when the sub-assembly 644, the upper device614 and the lower device 612 are clamped together. With this arrangementof hard solder balls, no underfill material or plastic caps are neededto keep the solder balls from being collapsed under pressure. Thesub-assembly 644 is soldered and cleaned, and is then ready to beconnected to a lower device 612 via the land grid array connector. Forthis, an interposer sheet 630 contains a pattern of holes 632 containingC-springs 664. This construction allows a thick interposer sub-assemblyto be constructed with internal ground and power planes to reduce thenoise and inductance that are inherent in long electrical leads. Aspreviously described, the C-springs can be replaced with other types ofelectrical connectors, such a wadded wire, without departing from thepresent invention.

[0041] The electrical connectors through the interposer may be selectedfrom a number of different types. Among these are metal filled polymers.Others are compressible wadded wires commonly referred to as fuzzbuttons. Metal springs, as previously described, may also be used.

[0042] The interposer is generally planar and is made from plastic orsimilar material having good mechanical strength and dimensionalstability. It usually is an insulator made from plastic material, suchas a polyphenylsulfide resin known as Ryton R-4 sold by Phillips 66Corporation, or a liquid crystal polymer, such as VECTRA E130i availablefrom Hoechst Celanese Corporation. The interposer serves to electricallyand mechanically isolate the area array devices from the printed wiringboard.

[0043] Turning now to FIG. 7, another embodiment is shown wherein athick interposer 730 includes a plurality of holes extendingtherethrough, filled with a conductive paste 790, such as a cured silverfilled elastomeric compound. Each hole is filled with a carefullycontrolled excess of the compound which is then cured or hardened. Uponhardening, the excess forms a contact button 792 on the top surface ofthe interposer and another contact button 794 on the bottom surface. Theinterposer is clamped between an upper area array device 714 and a lowerarea array device 712 to form a completed assembly. The various contactpoints on the two area array devices are not shown in the drawing. Theinterposer 730 and the contact buttons 792, 794 serve to maintain aminimum space of about 3 mm, for example, between the two area arraydevices.

[0044] The costs associated with the constructing and qualifying ofspecialized interposers is reduced by following the teachings of thepresent invention. Further, variations in size and thickness from oneinterposer to the other are reduced. The invention utilizes theadvantages of metal-to-metal contact throughout the system. It allowsthe use of solder joints in compression to improve their reliability.

[0045] The specific details and operation of the sub-assembly describedherein as well as the details of the various passive and active devicesthat are used here are known to persons of ordinary skill in the art.Accordingly, these details do not comprise a part of the presentinvention, except to the extent that they and their operation have beenmodified to become part of the present invention.

[0046] While the invention has been described in combination withembodiments thereof, it is evident that many alternatives,modifications, and variations will be apparent to those skilled in theart in light of the foregoing teachings. Accordingly, the invention isintended to embrace all such alternatives, modifications and variationsas fall within the spirit and scope of the appended claims.

What is claimed is:
 1. An electronic sub-assembly comprising a printedwiring board having a first surface including a plurality of viasterminating at electrical contacts at said surface, a first area arraydevice having a surface including a plurality of electrical contacts,and a plurality of connectors for electrically coupling the electricalcontacts on the surface of the printed circuit board to the electricalcontacts on the surface of the first area array device.
 2. Theelectronic sub-assembly according to claim 1 wherein the printed wiringboard has a second surface including a plurality of electrical contacts,said sub-assembly further including a second area array device having asurface including a plurality of electrical contacts, the sub-assemblyfurther including a plurality of connectors for electrically couplingthe printed circuit board to the second area array device.
 3. Theelectronic sub-assembly according to claim 1 wherein an interposerseparates the first surface of the printed wiring board from the surfaceof the first area array device, the interposer including the connectorsthat couple said printed circuit board to the first area array device.4. The sub-assembly according to claim 1 wherein the first area arraydevice is spaced a fixed minimum distance from the printed wiring board.5. The sub-assembly according to claim 4 wherein the minimum fixeddistance is about 3 mm.
 6. The sub-assembly according to claim 1 whereinthe connectors are selected from the group consisting of compressiblewadded wire contacts, metal springs, filled conductive elastomers,solder balls and hard solder balls.
 7. The sub-assembly according toclaim 3 wherein the connectors in the two interposers are selected fromthe group consisting of compressible wadded wire contacts, metalsprings, filled conductive elastomers, solder balls and hard solderballs.
 8. The sub-assembly according to claim 7 wherein the connectorsin the two interposers are selected from the same group.
 9. Thesub-assembly according to claim 7 wherein the connectors in oneinterposer are different than those in the other interposer.
 10. Thesub-assembly according to claim 1 wherein the electrical contacts on thefirst generally planar surface of the printed wiring board comprisecontact pads, the connectors are compressible, and the connectors ofsaid first interposer have a first end soldered to said contact pads anda second end compressed against the electrical contacts on the generallyplanar surface of the first area array device.
 11. The sub-assemblyaccording to claim 2 wherein the electrical contacts on the twogenerally planar surfaces of the printed wiring board comprise contactpads, the connectors are compressible wadded wire, the connectors ofsaid first interposer and the second interposer have a first endsoldered to said contact pads on the printed wiring board and a secondend compressed against the electrical contacts on the planar surface ofthe first and the second area array devices, respectively.
 12. Thesub-assembly according to claim 2 wherein the printed circuit boardcontains a plurality of plated through holes connecting the twosurfaces, and at least some of the electrical contacts on one or both ofthe generally planar surfaces of the printed wiring board comprisecontact pads that are offset from the corresponding holes in the printedwiring board to which the contact pads are electrically connected. 13.The sub-assembly according to claim 10 wherein the compressibleconnectors are wadded wire.
 14. A process for fabricating an electronicsub-assembly comprising the steps of: providing a printed circuit boardhaving a first surface and a plurality of vias terminating as electricalcontacts at said surface; providing a contact pad at the end of eachvia; providing an area array device having a surface facing the firstsurface of the board, said area array device having a plurality ofconductive input and output contacts on said surface; and providing aplurality of connectors to electrically interconnect the contact pads onsaid printed circuit board and the conductive contacts on said areaarray device.
 15. The process according to claim 14 including: providingan interposer structure between the printed wiring board and the areaarray device, the structure having first and second surfaces andcontaining a pattern of holes corresponding to the contact pads at theends of the vias on the surface of the printed wiring board; andincorporating each connector in a hole in the interposer structure. 16.The process according to claim 15 further including: joining one end ofa connector to each contact pad on the surface of the printed wiringboard using solder or a paste, and joining a second end of a connectorto each conductive contact on the surface of said area array device bycompression.
 17. The process according to claim 12 wherein the areaarray device is spaced a fixed minimum distance from the printed wiringboard.
 18. The process according to claim 15 further including:providing the printed wiring board with a second surface with the viasextending through from the first surface to the second surface to formplated through holes; providing contact pads electrically joined to theholes terminating at the second surface of the printed wiring board;providing an area array device facing the second surface of theinterposer, said device having a plurality of conductive input andoutput contacts corresponding in location to the wire contacts in thesecond interposer; providing a second interposer structure having firstand second surfaces between the second surface of the board and thesecond area array device, the structure containing a pattern of holescorresponding in location to the pads at the ends of the plated holes onthe second surface of the printed circuit board, and incorporating theconnectors in the holes in the structure.
 19. A printed wiring boardsub-assembly comprising: a printed wiring board having a first surfacecontaining one or more vias extending from the first surface into theboard, and a conductive contact pad on the surface of the printed wiringboard electrically joined to each of said vias; an interposer structurehaving a first surface facing the first surface of the printed wiringboard, and a second surface, a pattern of holes through the structurecorresponding to the holes in the printed wiring board, and electricalcontacts pressed into each hole in the pattern, each of said contactshaving a first end facing the first surface of the printed wiring board;said first end of each electrical contact of the first interposerstructure soldered or pasted to a contact pad on the first surface ofthe printed wiring board.
 20. The sub-assembly according to claim 19wherein each wadded wire contact has a second end, and the second end iselectrically coupled by compressive contact to a conductive lead on asurface of a module or another printed wiring board.
 21. Thesub-assembly according to claim 19 wherein the printed wiring board hasa second surface and at least some of the vias extend through the boardand terminate at the second surface at conductive contact pads on saidsurface, a second interposer structure having first and second surfaces,a pattern of holes corresponding to the contact pads on the secondsurface of the printed wiring board, and wadded wire electrical contactspressed into each hole in the pattern, with one end of each wadded wirecontact joined by soldering or pasting to the corresponding contact padon the second surface of the printed wiring board; and the other end ofeach wadded wire contact is electrically joined by compressive contactto a conductive lead on a surface of a module or another printed wiringboard.
 22. The sub-assembly according to claim 19 wherein the electricalconnectors are wadded wire.
 23. A method of making a printed wiringboard sub-assembly comprising: providing a first surface on the printedwiring board surface; providing one or more vias extending from thesurface into the board; providing an electrically conductive contact padon the first surface of the printed wiring board connected to each via;providing a first interposer structure having a first surface facing thesurface of the printed wiring board, and a second surface, saidinterposer including a pattern of holes corresponding to the vias in theprinted wiring board, and electrical connectors in each hole in thepattern; and joining a first end of each connector in the interposerstructure to a conductive pad on the first surface of the printed wiringboard using solder or paste.
 24. The method according to claim 23further including the step of electrically joining a second end of eachconnector by compressive contact to a conductive lead on a surface of anarea array device.
 25. The method according to claim 24 includingextending the vias through the printed wiring board to form platedthrough holes; providing an electrically conductive contact pad on thesecond surface of the printed wiring board connected to each of saidplated through holes; providing a second interposer structure whereinthe second interposer structure contains a pattern of holescorresponding to the contact pads on the second surface of the printedwire board; providing a connection in each hole in the pattern; andjoining a first end of each connector of the second interposer structureto each conductive pad on the second surface of the printed wiring boardby soldering or paste.
 26. The method according to claim 25 wherein asecond end of each connector in the second interposer is electricallyjoined by compressive contact to a conductive lead on a surface ofanother module or another printed wiring board.
 27. An electronicsub-assembly comprising a printed wiring board having a first surfaceincluding a plurality of vias extending from the surface into the board,a plurality of electrical contacts pads connected to each of the vias,at least some of the contact pads being offset from the correspondingvias in the printed circuit board to which the contact pads areelectrically connected, a first area array device having a surfaceincluding a plurality of electrical contacts, said two surfaces facingone another and spaced a fixed minimum distance from one another, andhaving a plurality of connectors for electrically coupling electricalcontacts on the surface of the printed circuit board to the electricalcontacts on the surface of the first area array device.
 28. Thesub-assembly according to claim 27 including a cap separating the twofacing surfaces, the interposer including said connectors.
 29. Thesub-assembly according to claim 27 wherein the minimum fixed distance isabout 3 mm.
 30. The sub-assembly according to claim 27 wherein theconnectors are selected from the group consisting of compressible waddedwire contacts, metal springs, filled conductive elastomers, solder ballsand hard solder balls.
 31. The sub-assembly according to claim 30wherein the connectors are hard balls consisting of copper balls coatedwith a layer containing tin.
 32. The sub-assembly according to claim 27wherein the connectors are solder balls embedded in an underfillmaterial filling the space between the printed wiring board and the areaarray device.
 33. The sub-assembly according to claim 28 wherein theprinted wiring board has one or more edges, and the plastic caps includea lip extending over an edge of the board.
 34. The sub-assemblyaccording to claim 33 wherein solder balls are used as connectors, andthe interposer has a thickness at least as great as the diameter of thesolder balls to prevent collapse of the solder balls.
 35. A method ofmaintaining a fixed minimum distance between a printed wiring board andan area array device comprising: providing a printed wiring board havinga first surface including a plurality of vias extending from saidsurface into the board; providing a plurality of electrical contactspads on said first surface connected to each of the vias, at least someof the contact pads being offset from the corresponding vias to whichthe contact pads are electrically connected; providing a first areaarray device having a surface including a plurality of electricalcontacts, said two surfaces spaced a fixed minimum distance from oneanother, and providing a plurality of connectors for electricallycoupling the contact pads on the surface of the printed circuit board tothe electrical contacts on the surface of the first area array device.36. The method according to claim 35 further providing an interposerseparating the two surfaces, the interposer including said connectors.37. The method according to claim 35 wherein the minimum fixed distanceis about 3 mm.
 38. The method according to claim 36 wherein theconnectors are selected from the group consisting of compressible waddedwire contacts, metal springs, filled conductive elastomers, solder ballsand hard solder balls.
 39. An electronic sub-assembly comprising aprinted wiring board having first and second generally planar surfacesand a plurality of vias terminating in electrical contact pads on thesurfaces; a first area array device having a generally planar surfacefacing the first generally planar surface of the printed wiring boardand having a plurality of electrical contacts thereon; a plurality ofconnectors for electrically coupling the electrical contacts on thefirst surface of the printed circuit board to the electrical contacts onthe surface of the first area array device, the connectors beingselected from the group consisting of wadded wire contacts, metalsprings, filled conductive elastomers, solder balls and hard balls; afirst interposer separating the first surface of the printed wiringboard from the surface of the first area array device a fixed minimumdistance of about 3 mm, the interposer containing a pattern of holescorresponding in location to the contacts on the first surface of theprinted wiring board, said connectors that couple said printed circuitboard to the second area array device being positioned in said holes;said sub-assembly further including a second area array device having asurface including a plurality of electrical contacts for electricallycoupling the second surface of the printed circuit board to the secondarea array device; a second interposer separating the second surface ofthe printed wiring board from the surface of the second area arraydevice, the interposer containing a pattern of holes corresponding inlocation to the contacts on the second surface of the printed wiringboard; a plurality of connectors positioned in the pattern of holes inthe second interposer for electrically coupling the electrical contactson the second surface of the printed circuit board to the electricalcontacts on the surface of the second area array device; at least someof the electrical contacts on one or both of the generally planarsurfaces of the printed wiring board comprising contact pads that areoffset from the corresponding vias in the printed wiring board to whichthe contact pads are electrically connected; and at least some of theconnectors in the holes of said interposers having a first end solderedto said contact pads and a second end compressed against the electricalcontacts on the generally planar surface of the corresponding area arraydevice.
 40. The sub-assembly according to claim 39 wherein the secondinterposer separates the second surface of the printed wiring board afixed minimum distance of about 3 mm from the surface of the second areaarray device.
 41. A process for fabricating an electronic sub-assemblycomprising the steps of: providing a printed wiring board having a firstand a second generally planar surface and a plurality of viasterminating as electrical contact pads at each said surfaces; providinga contact pad on the surface at the end of each via; spacing a firstarea array device having a generally planar surface facing the firstsurface of the board a fixed minimum distance of about 3 mm from theprinted wiring board, said area array device having a plurality ofconductive input and output contacts on said surface, at least some ofwhich correspond to the contact pads on the first surface of the printedwiring board; providing a first interposer between the printed wiringboard and the first area array device, said interposer structure havingfirst and second generally planar surfaces and containing a pattern ofholes corresponding in location to the contact pads at the ends of thevias on the first surface of the printed wiring board; providing aplurality of connectors to electrically interconnect the contact pads onsaid printed circuit board and the conductive contacts on said firstarea array device; incorporating each connector into one of the holes inthe first interposer structure; providing a second area array devicehaving a generally planar surface facing the second surface of theprinted wiring board, said second area array device having a pluralityof conductive input and output contacts, at least some of whichcorrespond to the contact pads on the second surface of the printedwiring board; providing a second interposer between the second surfaceof the printed wiring board and the second area array device, saidinterposer having first and second surfaces and a pattern of holescorresponding in location to the contact pads at the ends of the platedholes on the second surface of the printed circuit board and thecorresponding conductive input and output contacts on the second areaarray device; incorporating electrical connectors into the holes in saidsecond interposer; soldering or pasting one end of at least some of theconnectors of said interposers to said contact pads and using pressurecontact to join the other second end against the electrical contacts onthe generally planar surface of the corresponding area array device. 42.The process according to claim 41 wherein the second interposerseparates the second surface of the printed wiring board a fixed minimumdistance of about 3 mm from the surface of the second area array device.